1. Field of the Invention
The present invention relates to a power supply unit for supplying electric power to a load circuit.
2. Description of the Prior Art
FIG. 6 shows a conventional constant-voltage power supply unit. As shown in FIG. 6, this power supply unit has a pnp-type output transistor Tr, a first resistor R1, and a second resistor R2 connected in this order between the power source line 6 supplying a voltage Vcc and ground (reference potential point), with the node a between the collector of the output transistor Tr and the first resistor R1 connected to an output terminal 4.
The node b between the first and second resistors R1 and R2 is connected to the non-inverting input terminal (+) of a comparator 3. The inverting input terminal (-) of the comparator 3 is connected to the node d between one end of a resistor R.sub.0 and a starting switch 2. The other end of the resistor R.sub.0 is connected to ground, and the other end of the switch 2 is connected by way of a constant current source 1 to the power source line 6 supplying the voltage Vcc. It is assumed that the voltage at the node d is V1 when the starting switch 2 is turned on. To the output terminal 4, a capacitor 5 is connected to prevent oscillation of the power supply unit.
In reality, the capacitance of the load 7 that is connected to the output terminal 4 is additionally connected in parallel with the capacitor 5. However, for simplicity's sake, it is assumed here that the capacitor 5 includes the capacitance of the load 7. The power supply unit having the above-described construction starts operating when the switch 2 is turned on.
As soon as the switch 2 is turned on, the voltage V1 at the node d is applied to the inverting input terminal (-) of the comparator 3. As a result, the comparator 3 outputs a low level and thereby turns on the transistor Tr, allowing the capacitor 5 to be charged quickly. When the voltage at the node b exceeds the voltage V1, the comparator 3 outputs a high level and thereby turns off the transistor Tr. However, as an electric current is kept supplied to the load 7, the voltage at the node b drops, until eventually the transistor Tr is turned on again. Thus, the output transistor Tr operates in such a way that the voltage at the node b (thus the voltage at the output terminal 4) is kept constant.
In this conventional power supply unit, just when it is started up by turning on the switch 2, a predetermined maximal current flows through the capacitor 5. As the characteristic curve shown in FIG. 7 indicates, the current Imax that flows at the start-up is larger than the current Io that is ordinarily supplied to the load, and thus the output current varies through points A, B, C, and D on the curve in this order before it settles down to point D.
Inconveniently, this causes the source of the output current, i.e. the power source line 6, to become unstable. As a result, other circuits 8 and 9 that are also connected to the same power source line 6 may malfunction. For example, if the circuits 8 and 9 include a microcomputer, there is a possibility that the microcomputer is reset unexpectedly.